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FAMENA issue 5, volume 42, Zagreb 2018
Mario Holik, Marija Živić, Zdravko Virag, Antun Barac
Comparison of Finned Tube and Plate-Finned Heat Exchangers in Waste Heat Recovery
The use of waste heat recovery devices on mobile units (trucks and ships) is usually limited by the available space and the application of compact heat exchangers is recom- mended for such purposes. The performance of the heat exchanger is defined by the optimized Rankine cycle (to achieve maximum power) and it depends on the mass flow and temperature of the flue gases and selection of the working fluid in Rankine cycle. An example of selection of the preheater performance (in case of water as the working fluid) is considered here, wherein the several surface types of finned tube and plate-finned heat exchangers are used, for which there are measured data of the heat transfer coefficient and friction factor. Heat exchangers are sized according to the criteria of maximum allowed velocity of the flue gases and shapes of the heat exchanger frontal area. The sized heat exchangers were compared with respect to the heat transfer coefficient, the area of the heat exchanger surface on the finned side, the volume of the heat exchanger and the pressure drop on both sides. From the comparison of the best plate-finned and the best finned tube heat exchangers it is concluded that in the recommended range of flue gases velocity (from 4 m/s to 6 m/s) the pressure drop at the gas side are similar (in the plate-finned heat exchanger it is in the range from 53.5 to 112 Pa and in the finned tube exchanger from 53.8 to 142.8 Pa), while the plate-finned exchanger has more than 50% smaller heat transfer area, compared to the finned tube one.
Key words: Heat exchangers, Heat transfer, Minimization of heat exchanger volume, Pareto frontier
Martina Rauch, Antun Galović, Nenad Ferdelji, Saša Mudrinić
Local Entropy Production of the Parallel Flow and Counterflow Heat Exchanger
The paper gives an analytical dimensionless analysis of the local entropy generation and its ratio with the local exchanged heat flow rate for parallel flow and counterflow heat exchangers. The end (side) of the heat exchanger, which is the inlet of the weaker stream, is, among other, proven to be relevant variable for those local values. Elaborated algorithm provides explicit connection between entropy generation and ratio of entropy generation to local exchanged heat flow rate in dependence on the relevant dimensionless variables A/A0, π2 = kA0/C1, π3 = C1/C2 and πT=T1'/T2'. Value π2 vary between 1.0 and 4.0, value π3 amounts 0.0; 0.5 and 1.0, while 0.5 and 2.0 were taken for value πT. From obtained general equations for parallel flow and counterflow heat exchanger special cases for observed values were extracted, considering the cases where one of the streams condenses or evaporates. Given algorithm, aside the local amounts of the observed values, also gives overall amounts of the mentioned values. The results are presented by appropriate diagrams and additionally interpreted.
Key words: parallel flow and counterflow heat exchanger, local entropy generation, local rate of heat flow, dimensionless analysis
Jura Tomorad, Ivan Horvat, Damir Dović
Study of Operative Temperature Using the Novel Detail Approach in Determining Mean Radiant Temperature – Comparison Between Wall-Mounted Convector and Conventional Radiator
Most of the physical parameters that are used to assess the satisfaction with the ambient thermal condition in a mathematical way are contained within the definition of operative temperature. This temperature, which can be used as a representative of indoor thermal comfort, is a function of the air temperature, the mean radiant temperature and the relative air velocity.
In this paper, the room air, mean radian temperature and indoor air velocity were determined experimentally for wall-mounted convector and conventional radiator at controlled room conditions. The room air temperature and indoor air velocity were continuously measured at several positions and heights (0.75 m and 1.5 m) using calibrated T-type thermocouples and hot wire probes, while mean radiant temperature was calculated using the thermograms captured by the IR thermal camera and numerically computed radiation view factors. Each wall was divided into several sections with approximately similar temperatures (differences < 0.5 °C) for which view factors were determined. Thermal heat output of the tested heat emitters was derived according to EN 442-2:2014. Obtained results were analysed and conclusions about the achieved thermal comfort and related energy saving were made accordingly.
Key words: Mean radiant temperature, operative temperature, thermal comfort, thermography, heat emitters
Luka Boban, Vladimir Soldo, Jure Stošić, Eugen Filipović, Filip Tremac
Ground Thermal Response and Recovery after Heat Injection: Experimental Investigation
Monitoring the ground thermal response to a constant heat flux input is common method for determination of effective ground properties needed for sizing the ground coupled heat pumps. In this work, the experimental procedure included two TRT's with different average injection heat fluxes, 4.43 kW and 7.64 kW, applied to the same borehole. Recorded temperatures of fluids, circulated in an experimental borehole heat exchanger U-tube, are used to determine the ground thermal conductivity and the borehole thermal resistance with the infinite line source model (ILS). Additionally, thermocouples placed on the borehole wall up to the depth of 100 m enabled the measurement of temperature profiles of undisturbed ground and during the recovery period between the two TRT's. The results indicate that true undisturbed state after injected heat flux cannot be reached in short time while the use of higher injection heat flux reduces the influence of the ground's inhomogeneity on the results obtained.
Key words: borehole heat exchanger, thermal response test, ground thermal properties, borehole thermal resistance
N. Bilandžija, N. Voća, J. Leto, V. Jurišić, M. Grubor, A.Matin, A. Geršić, T. Krička
Yield and Biomass Composition of Miscanthus x Giganteus in the Mountain Area of Croatia
Although biomass of Miscanthus x giganteus shows a significant potential for production of second-generation biofuels, it is currently mostly used as a combustion fuel. The objective of this paper is to investigate: (I) dry matter yield and yield components; (II) biomass composition; and (III) potential divergences of the investigated parameters from the standard for solid fuels CEN/TS 14961:2005, in relation to two harvest seasons and six fertilizer treatments. The investigation has determined that there is a potential for producing significant quantity of biomass from M x giganteus in the investigated agro-ecological conditions of the mountain areas of Croatia. The laboratory analyses indicated the suitability of using biomass in direct combustion.
Key words: Energy crop, Miscanthus x giganteus, dry matter yield, combustion properties
Šefik Behrem, Bahrudin Hrnjica
Estimate of Heat Transfer Coefficient during Quenching Steel in Water
The paper presents the process of two-dimensional axisymmetric quenching of cylindrical samples in water at 40 °C. Experimental work consists of quenching three dimensionally different cylindrical probes. The dimensions of the probes are: Φ25×50,Φ50×150 and Φ75×225 mm. Three measuring points are 1.5 mm below the cylinder surface positioned at the cylinder height, whereas the fourth measuring point is in the center of gravity of the cylinder. The quenching was conducted in strict controlled rate of water flow at the cylinder head. The problem of the task belongs to inverse problems of heat conduction, or ill posed problems, so that the solution to the problem leads to a sufficiently accurate estimate of the unknown heat transfer coefficient, imposed on the outer surface of the cylinder. Computer model solutions use experimental results of temperature measurements to minimize errors between computer calculations and measured temperature values in the same place. The selected solution algorithm is a hybrid algorithm that consists of a combination of three different algorithms of solution, connected into a single unit.
Key words: Cylindrical probe, steel quenching, optimization, inverse heat transfer, heat transfer coefficient
Ivan Horvat, Damir Dović
Combustion of Agricultural Biomass - Issues and Solutions
Biomass is one of the oldest energy source known to mankind. Progress made so far in understanding of a rather complex direct combustion process allowed for a constant technology development. Increasingly stringent environmental requirements of the EU, especially in terms of the pollutant emissions, require development of new design solutions of the combustion system for residential hot water boilers fuelled by some form of solid biomass. The emissions are becoming even greater problem if, various residues from agricultural production are used instead of a wooden biomass, as they are becoming more desirable fuel due to their lower cost.
In this paper, various issues related to the combustion of agricultural biomass are discussed. Attention is given to the problems associated with high volatile matter contents, presence of nitrogen, sulphur, chlorine and low ash melting temperatures. At the end, novel design modifications of a conventional residential biomass combustion system are proposed, as the well-known methodologies for combustion and emissions control applied in industrial- scale boilers are in most cases not suitable for small-scale units.
Key words: Agricultural biomass combustion, pollutant emissions, ash related problems, dioxin emission
Marko Pirc, Jurij Avsec, Nataša Čelan Korošin, Urška Lavrenčič Štangar, Romana Cerc Korošec
Cable Aging Monitoring with Differential Scanning Calorimetry (DSC) in Nuclear Power Plants
As a requirement for plant life extension for more than 40 years, additional Cable Aging Management Program (CAMP) has to be implemented in Nuclear Power Plant Krško. Samples of cables are selected based on nuclear safety and electrical equipment criticality for inspection and testing, to check functionality and prevent unexpected failure during normal operation. Different onsite testing equipment and methods are implemented to find harsh environment due to temperature, radiation, humidity and chemical effects that could affect insulation lifetime. Infrared thermography is used for determining and evaluating temperature hot spots.
The article presents a development of laboratory testing of cable insulation using Differential Scanning Calorimetry (DSC). Thirty-six samples of different nuclear qualified cables made of most frequently used materials, ethylene propylene rubber (EPR) and cross linked polyethylene (XLPE) – all with chlorosulfonated polyethylene (CSPE) jacket, were tested. Samples were 35 years old and additionally temperature aged in several steps with an intention to get acceptance criteria. Similar tests were conducted in two testing laboratories.
The results showed an evident decrease in oxidation stability of the inner EPR insulation; the onset temperature of oxidation processes has been shifted from 238 °C (unaged samples) to 175 °C (most aged samples). A decrease in oxidation stability was also observed for XPLE insulation; the onset oxidation temperature decreased from 266 °C for unaged samples to 213 °C (most aged samples). For the jacket material CSPE used as the insulation protection nearly no changes were observed.
Key words: Cable ageing, cable maintenance, nuclear technology, laboratory testing, DSC, thermal calorimetry
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